This invention relates to pyroelectric radiation detector devices and circuit arrangements including a pyroelectric radiation detector device. More particularly, but not exclusively, the invention relates to such devices and circuit arrangements in which the detector device comprises a pair of detector elements formed of polarized pyroelectric material in which the elements have been polarized by subjecting the pyroelectric material of the elements to a high electric field at an elevated temperature.
The use of pyroelectric material for radiation detection, in particular infrared radiation detection, is well established. The pyroelectric effect is the change of the electrical polarization of a crystal due to a temperature change. The state of polarization is not generally observable because under equilibrium conditions it is compensated by the presence of free charge carriers that have reached the surface of the crystal by conduction through the crystal and from the outside. The magnitude of the polarization, and hence of the compensating charge, depends on the temperature. When the temperature is changed so that the supply of compensating charges is less than the variation in the polarizing charge, then the crystal surfaces acquire an observable charge. This effect is employed in detector devices by making a body of pyroelectric material into a capacitor with electrodes on oppositely located plane surfaces which are perpendicular to the direction of polarization of the material. The redistribution of the compensating charges causes a current to flow in an external circuit to which the detector is connected.
For use in the detection of infrared radiation, for example for use in intruder detection equipment, various pyroelectric materials have been employed. These include materials such as triglycine sulphate, modified lead zirconate titanate, lithium tantalate and certain plastic film materials such as polyvinylidene fluoride. Polarization is normally achieved in such materials by applying an electric field in the direction of the polar axis, sometimes while simultaneously subjecting the material to an elevated temperature, so as to align the electric dipoles. In some materials, for example L-alanine doped triglycine sulphate, it is not necessary to induce the polarization by application of an electric field as full polarization already exists in the material.
For use in intruder detection equipment it is possible to employ a detector device comprising a single detector element of pyroelectric material and various detector devices are commercially available for this purpose. One such detector device is described in United Kingdom patent application No. 16110/77, corresponding to U.S. Pat. No. 4,198,564. However it has also been found that for the purpose of detecting small movements across the total field of view of the detector device an increased sensitivity can be obtained when the detector device comprises more than one detector element. In this configuration the device can be constructed so that uniform changes in input radiation in the fields of view of all the elements, for example changes in background radiation, will produce voltages across pairs of elements which are in opposition and therefore no net signal voltage is created. On the other hand, a change in input radiation in the field of view of one element but not in the field of view of another element can produce a differential output signal.
One such known radiation detector device comprises a pair of detector elements arranged in a common plane. Each element is formed of a body or body part of polarized pyroelectric material. The elements each have electrodes on opposite major surfaces in overlapping relationship and the electrodes extend substantially normal to the direction of polarization of the pyroelectric material. The elements are electrically connected within the devie via their electrodes to form two series-connected capacitive detectors in which the directions of polarization of the pyroelectric materials are in opposition.
Hereinafter, any reference to the directions of polarization of the elements being in opposition is to be understood to mean that in the series connection the signal polarities produced by the individual elements under irradiation are in opposition for a common radiation input.
In such a device the structure may be obtained using a single body of pyroelectric material having first and second main electrodes on one major surface and a common electrode on the opposite major surface of the body, the common electrode being in overlapping relationship with both of the first and second main electrodes. In the normal detector operation no electrical terminal connection is made to the common electrode, the first and second main electrodes each having a terminal connection. It is of course alternatively possible to modify the structure and employ separate bodies with an appropriate common connection between the two elements to give the desired series connection of the two capacitive detectors.
Detector devices comprising two pyroelectric elements connected in series may be referred to as "Dual" detectors. The operation of a "Dual" detector, the directions of polarization of the two elements are in opposition, is based on the principle that where there is the same temperature change in both elements due to the same radiation change in the fields of view of both elements, due to their differential electrical connection the voltages generated across the capacitive detectors will cancel. When, however, the change in temperature of one element, as determined by the change in radiation in the field of view of the one element, is not accompanied by a corresponding change in temperature of the other element, as determined by the change in radiation in the field of view of the other element, a differential signal voltage is created. It has been found that the use of such "Dual" detectors is highly suitable in intruder detector systems. One important advantage is that fluctuations in the thermal state of the background scene produce no output signal from the detector. In single element detectors such fluctuations produce a noise-like signal, sometimes referred to as environmental noise. This noise is substantially reduced in the alarm circuitry of "Dual" detectors, thereby giving a greater range of detection. Alternatively, the range can be kept the same with the result that the probability of a false alarm is substantially reduced. Clearly a compromise between these two extremes can be chosen if desired.
A further advantage lies in the fact that a change in the temperature of the dual element detector due to an ambient temperature change will result in a much lower change in output compared with the change that would occur with a single element detector. The level of such a change in output of a dual detector depends on the degree of matching of the two elements. Hence the dual detector is more tolerant of changes of environmental temperature within the alarm circuit enclosure.
A known circuit arrangement including a "Dual" detector includes signal amplifying means, for example field effect transistor (FET) amplifier means. The input circuit of the FET amplifier means including the series connected capacitive detector elements.
When using certain ceramic pyroelectric materials, for example modified lead zirconate titanate, in the "Dual" detectors, the pyroelectric material is polarized by subjecting the material, normally after defining the elements therein, to a high electrical field at an elevated temperature. Certain problems may arise with these detectors in operation insofar as their ability to withstand temperature variations over a wide range is concerned. One difficulty involved when subjecting the elements to wide temperature variations, for example from -40.degree. C. to +100.degree. C. is that in certain pyroelectric materials the polarization may be diminished even after a few of such temperature excursions. This depoling property is to a certain extent related to the choice of material. The temperature variation induced depoling effect is greater when the maximum temperatures concerned are nearer the Curie point of the material. Thus for a material such as lithium tantalate, where the Curie point is approximately 600.degree. C., the effect is potentially less troublesome than with a material such as modified lead zirconate titanate where the Curie point may be 100.degree. C. or lower. However, for other technological reasons it is undesirable that the choice of the pyroelectric material for the detector elements should be determined by the ability of the material to withstand wide temperature variations without depoling.